Production of ordinary superphosphate with strong sulfuric acid



Feb. 23, 1965 PHOSPHATE 1 GRIND (75-95% --200 MESHI L. D- HAND, JRPRODUCTION OF ORDINARY SUPERPHOSPHATE WITH STRONG SULFURIC ACID FiledNOV. 6. 1961 SULFURIC ACID (96% OR HIGHER) REOYOLE, IF DESIRED SCREENADD WATER (GRANULATEI DRYING CURING DEN 30 MINUTES REQUIRED FOR FLUORINETO EVOLVE (OPTIONAL) WITH WARM (lOO-I50' EI AIR FOR ABOUT 4 MINUTESIMPROVES PHYSICAL CONDITION BY REDUCING STICKINESS; NOT NECESSARY FORDESIRED CHEMICAL RESULTS.

3 TO 7 WEEKS REQUIRED TO GIVE 6000 F 0 AVAILABILITY AND LOW MOISTURECONTENT Wm5%m%mm United States Patent United States Filed Nov. 6, 1961,Ser. No. 150,600 2 Claims. (Cl. 71-40) (Granted under Title as, us. Code1952), see. 266) .The invention herein described may be manufactured andused by or for the Government for governmental purposes without paymentto me of any royalty thereon. 7 My invention relates to a method ofpreparing granular ordinary superphosphate from strong sulfuric acid,and more particularly to a method for the production of granularsuperphosphate wherein the superphosphate material is granulated priorto curing, thereby eliminating the necessity of subsequently blasting ordigging the material out of a curing pile.

The fertilizer industry has recognized the trend toward the desirabilityof producing fertilizer in granular form due to the need for betterphysical condition of high-analysis grades. Granulation of fertilizermaterial reduces caking, decreases dustiness, provides for easier andmore uniform distribution, and results in a more attractive appearanceof the product. Because of these benefits gained with granularhigh-analysis fertilizer grades, farmers are beginning to demandgranulation of all solid fertilizers, including ordinary superphosphate.Furthermore, granulation of this fertilizer materialis highly desirableif the superphosphate is intended for blending with other granularmaterial.

' Heretofore it has been the practice in the fertilizer industry, whenproducing granular ordinary superphosphate, to first obtain a producthaving desirable P 0 availability by curing phosphate rock treated withconventional 70 percent sulfuric acid in a large, exposed pile andsubsequently granulating the material by spraying Water on the curedproduct in a rotating drum. The superphosphate material dries out andsets up into an extremely hard mass while curing in the pile. Such setupmaterial must be removed from the pile and sized prior to thegranulation step. Due to the'extreme hardness of the material, theremoval from the pile often proves to be very diificult and requires theexpenditure or considerable energy which may be in the form of actuallyblasting the material with explosives. Furthermore, because of thedryness of the cured product, considerable water must be added to obtaingranulation and, even then, granulation has proved to be difiicult andhard to control. Such processes of the prior art result in the formationof substantial quantities of both oversize material and fines.Considerable drying must be accomplished to remove the excess Waterandto effect the strengthening of the granules that are formed. Theoversize material must be crushed, and both the oversize and fines mustbe recycled. Obviously, these prior-art processes tor the production ofgranular ordinary superphos 3,17%,784 Patented Feb. 23, 1965 powderyacidulate for a period of approximately 30 minutes to eifect fluorineremoval therefrom and then adding Water to the acidulate in a rotarydrum, wherein it is granulated. Furthermore, several new andadvantageous features over conventional processes for the production ofsuperphosphate are realized by the present invention.

Among these advantageous features are extremely desirable high P 0availability, good physical properties, low moisture content, and lowfluorine content in the material produced.

It is therefore an object of the present invention to provide a processfor the economical production of granular ordinary superphosphate whichsubstantially eliminates the necessity for curing a superphosphatematerial in large storage piles prior to granulation.

Another object of the present invention is to provide a process for theeconomical production of granular ordinary superphosphate whichsubstantially eliminates the necessity for curing a superphosphatematerial in large storage piles prior to granulation, therebyeliminating the formation of large piles of extremely hard, set-upmaterial, which require large expenditures of energy for the formationof pulverulent material therefrom.

Still another object of the present invention is to provide a method forthe production of granular ordinary superphosphate in which the recyclerate of fines and crushed oversize material is substantially reduced,thereby greatly increasing the capacity of existing process equipment.

A further object of the present invention is to provide a process forthe production of ordinary granular superphosphate material in which alarge proportion of the fluorine in the rock is caused to volatilizeduring acidulation.

A still further object of the present invention is to provide a methodfor the production of ordinary superphosphate fertilizer material inwhich approximately 3 times as much of the fluorine content in thephosphate rock treated is volatilized therefrom as compared to standardprior-art processes for making granular superphosphate material.

An additional object of the present invention is to provide a method forthe production of ordinary granular superphosphate in which the product,after curing, has desirable P 0 availability, good physical properties,and low moisture and fluorine contents. i g

In carrying out the objects of my invention in one form thereof, I havediscovered that a powdery, nonstickable' acidulate is formed when finephosphate rock and strong sulfuric acid of concentration in the range ofabout 96 percent to 99 percent are mixed. 1 have observed that thephysical properties of this acidulate are conducive to very eflicientgranulation. In carrying out my process, a closely sized granularproduct can be produced by spraying water on the above-mentioned powderyacidulate in a rotating drum. Sufiicient water can be added to thematerial in the granulation step to raise the moisture content ofcrushed product to the 4 to 5 percent required to obtain desirableconversion of the P 0 to an available form. In addition it has beenfound that, if desired, the acidulate may be held for a period of timebefore adding the Water to permit the fluorine to evolve. Also, the P 0availability obtained after curing has been observed to be as good as orbetter than that obtained in the conventional production of ordinarysuperphosphate, and the fluorine evolution is increased to 50 to 60percent greater than that obtained in any conventional processes of theprior art.

Y My invention, together with further objects and ad- 3 vantagesthereof, will be better understood from consideration of the followingdescription taken in connection with the accompanying drawing in which:

The figure is a flowsheet illustrating the principles of my novelprocess which results in granular superphosphate fertilizer having theproperties mentioned above.

Referring now more specifically to the figure, phosphate rock is groundto 75 to 95 percent minus-200 mesh. Sulfuric acid of 96 percentstrength, or higher, is mixed with the ground phosphate rock, and theresulting powdery acidulate is held for a period of approximately 30minutes, in which time the fluorine is evolved. After the short holdingperiod, the acidulate is granulated in a simple, horizontally inclinedrotating drum by spraying water on same in quantity suflicient to dilutethe acid content to about 80 percent H 50 The granulated material isthen directly passed on to the curing step or, preferably, it may beexposed to warm air (100 F. to 150 F.) for a period of about 4 minutesto improve its physical condition by reducing moisture content.

Any batch or continuous mixer currently used in ordinary superphosphateplants would be satisfactory. Obtaining the powdery, nonsticky materialhas been found to depend primarily on the acid concentration and theparticle size of the rock. When using sulfuric acid having an H 80content of 96 percent, it is necessary to size the rock to 94 to 96percent through 200 mesh. It would be necessary to grind the rock evenfiner if a more dilute acid were to be used. Thus, 96 percent acid,which is easily made in contact sulfuric acid plants, is about thelowest concentration practical due to the difliculty in grinding therock finer than 200 mesh.

It has been found, conversely, that somewhat coarser rock may be used byincreasing the acid concentration. The standard particle size, 70 to 80per cent minus-200 mesh, may be used with fuming acid (15 to 18 percentfree S It has been found that both the P 0 availability of the finishedproduct and the evolution of fluorine from the rock during acidulationare affected by the acid concentration. The P 0 availability is about 2percentage points lower when using fuming acid to 18 percent free S0than when using 96 percent acid. The P 0 availability also varies withthe grade of the rock when using particle sizes of about 95 percentminus-200 mesh; it is 2 to 3 percentage points higher when high-graderock is used as opposed to low-grade rock. The P 0 availability in theproduct produced from high-grade rock is increased by decreasing theparticle size from about 75 to 95 percent minus-200 mesh; but withlow-grade rock as raw material the P 0 availability is not aflectedappreciably. The evolution of fluorine increases 40 to 50 percent ormore when using the strong acid rather than the conventional 70 percentsulfuric acid. The exact amount depends upon the acid concentration, andtype and grade of phosphate rock used. It has been found thatapproximately 30 minutes is sufficient to evolve the fluorine before thewater-addition granulation step.

The amount of water which is added to the granulation step should bethat necessary to give a product with a moisture content of about 4 to 8percent. If the moisture content is less than about 4 percent,conversion of P 0 to an available form will be too slow. On the otherhand, if the moisture content is higher than about 8 percent, it hasbeen found that the material will overagglomerate in the granulator andthe moisture level of the cured product will be so high that the gradewill be un desirably reduced. Thus, by keeping the moisture content inthe 4 to 8 percent range, good granulation can be obtained and theproduct will not agglomerate severely, even with no recycle. Warm (100F. to 150 F.) air may be used in carrying out the process in one formthereof to surface dry or caseharden the fresh product granules. Such aprocedure will permit the use of part of the product as recycle or toimprove the physical condition, if desired. It has been found that onlya few minutes of exposure of the fresh product to the warm air isnecessary to effect the surface drying thereof.

After granulation and/or drying of the product, it has been foundnecessary to cure the fresh granules to obtain a desirable end product.The curing time required varies from about 2 to 7 weeks. After propercuring, the P 0 availability in the granular product has been found tobe percent or higher, and the moisture is reduced to less than about 1percent.

In order that those skilled in the art may better understand how thepresent invention can be practiced, the following examples are given byway of illustration and not by way of limitation.

EXAMPLE I Low-grade (32 percent P 0 Florida phosphate pebble ground to96 percent through 200 mesh was acidulated with 96 percent sulfuricacid, the quantity of acid added being sufficient to yield a mole ratioof (P O +SO to lime of about 1:1. The acidulate was held for 30 minutes,during which time 54 percent of the fluorine in the rock was evolved.After the 30-minute period, the material was granulated in a rotatingdrum by spraying water on it in quantity sufficient to dilute the acidto about 80 percent H SO About 70 percent of the product was minus 6-plus 20-mesh material. After curing for 3 weeks at F. in a sealedcontainer, the available P 0 content was 20.3 percent and the moisturecontent was 0.9 percent.

EXAMPLE II The same procedure was followed as in Example I above, exceptthat high-grade (35 percent P 0 Florida pebble ground to 94 percentminus 200 mesh was used, and after granulation the fresh product wascasehardened for 4 minutes. About 45 percent of the fluorine in the rockwas evolved during the 30-minute holding period. About 86 percent of thegranular product was minus 6- plus 20-mesh material. The moisturecontent of the product immediately after drying was 4.8 percent. Aftercuring for 7 weeks at 150 F., the available P 0 content was 21.0 percentand the moisture content was 1.2 percent.

EXAMPLE III The same procedure was followed as in Example II above,except that fuming acid (15 to 18 percent free S0 was used. The granuleswere not casehardened and the mole ratio of (P O +SO to lime was 1.02:1.During the holding period 49 percent of the fluorine was evolved. About78 percent of the product was minus 6- plus 20-mesh material. Aftercuring for 5 weeks, the available P 0 content was 21.4 percent and themoisture content was 2.0 percent.

EXAMPLE IV A coarser (77 percent minus-200 mesh) high-grade phosphaterock was substituted for the fine rock used in Example III above. Thefluorine evolution and the available P 0 content were the same as inExample III; however, the available P O was only 93 percent of thetotal, versus 97 percent of the total shown in Example H. The moisturecontent of the product was 1.0 percent. About 74 percent of the productwas minus 6- plus 20-mesh material.

EXAMPLE V The same procedure was followed as in Example IV above, exceptthat about 6 percent more acid was used. During the holding period 52percent of the fluorine was evolved. About 77 percent of the product wasminus 6- plus ZO-mesh material. After curing for 5 weeks, the P 0content was 21.4 percent, the available P 0 was 96 percent of the total,and the moisture content was 1.0

5 percent. The free acid content, 1.6 acceptable range.

Best conditions are summarized below:

Acid concentration 96 percent H SO Rock particle size 95 percent minus200 percent, was in an mesh. Denning time 30 minutes. Water added Togive equivalent acid concentration of about 80 percent H 80 productmoisture contentof 6 per cent to 8 percent.

Drying Warm (1G0150 F.) air;

. for'4 minutes. V Curing period 3 to 7 weeks.

The results of the several tests described in Examples I-V above areshown in the following table.

to yield a final product having a moisture content in the range fromabout 4 to 8 weight percent; granulating the resultingpowdery acidulate;and transferring the resulting granules directly to storage'for a periodof about 3 to 7 weeks until final equilibrium conditions are attained,said process being characterized by the fact that said granules have a Pavailability greater than about 96 percent by minutes to evolve fluorinetherefrom, said acidulate resulting from the acidulation of said finephosphate rock by said sulfuric acid and said acidulate characterized asa substan- Table 1 I Rock analy- Product analysis, percent sis, percentAcid, Fluorine Test percent Chemical, P 05 evolu- No. H1804 Screen,tion, 200 H 6 +20 percent P20 mesh Total A.P.A. Availaa mesh bility 1During holding period. 2 Fuming acid.

While I have shown and described a particular embodiment of myinvention, modifications and variations thereof will occur to thoseskilled in the art. I Wish it to be understood, there-fore, that theappended claims are intended to cover such modifications and variationswhich are within the true scope and spirit of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A'process for the production of granular ordinary superphosphatefertilizer material which comprises acidulating in a first vessel finephosphate rock of more than about 90 percent'minus-ZOO mesh withsulfuric acid of a concentration greater than about'96 percent H 80 thequantity of said acid added being'suflicient to yield a mole ratio inthe final product of (P O +SO to lime of about 1:1; holding in a secondvessel the resulting powdery acidulate for a period of about 30 minutesto evolve'at least about percent of the fluorine originally present in'7 said phosphate rock, said acidulate resulting from the acidulation ofsaid fine phosphate rock by said sulfuric acid and said acidulatecharacterized as a substantially dry and non stickable discreteparticulate mass; subsequently adding water to the resulting powderyacidulate in quantity sufiicient to dilute said acid to about 80 percentH SO the total quantity of water introduced into said resulting powderyacidulate (including that added indirectly through the addition of waterpresent in said sulfuric acid and that added directly to said resultingacidulate) beingsuflicient tially dry and non-stickable discreteparticulate mass; subsequently adding water to the resulting powderyacidulate in quantity sufiicien't to dilute said acid to about percent H80 the total quantity of water introduced into said resulting powderyacidulate (including that added indirect- V 1y through the addition ofwater present in said sulfuric acidand that added directly to thepowdery acidulate) being sufiicient to yield a final product having amoisture 7 References Cited by the Examiner UNITED STATES PATENTS2,504,546 4/50 Wight et al. 7140 2,680,679 6/54 Harvey et a1 71-372,844,455 7/58 Harris 71-37 X 2,882,127 4/59 Le Baron 23-109 2,906,6029/59 Purvis 23109 3,041,158 6/62 Boylen et a1. 71-40 DONALL H.SYLVESTER, Primary Eramine'r.

GEORGE D. MITCHELL,.ANTHONY SCIAMANNA, A. LOUIS MONACELL, Examiners.

1. A PROCESS FOR THE PRODUCTION OF GRANULAR ORDINARY SUPERPHOSPHATEFERTILIZER MATERIAL WHICH COMPRISES ACIDULATING IN A FIRST VESSEL FINEPHOSPHATE ROCK OF MORE THAN ABOUT 90 PERCENT MINUS-200 MESH WITHSULFURIC ACID OF A CONCENTRATION GREATER THAN ABOUT 96 PERCENT H2SO4,THE QUANTITY OF SAID ACID ADDED BEING SUFFICIENT TO YEILD A MOLE RATIOIN THE FINAL PRODUCT OF (P2O5+SO3) TO LIME OF ABOUT 1:1 HOLDING IN ASECOND VESSEL THE RESULTING POWDERY ACIDULATE FOR A PERIOD OF ABOUT 30MINUTES TO EVOLVE AT LEAST ABOUT 50 PERCENT OF THE FLUORINE ORIGINALLYPRESENT IN SAID PHOSPHATE ROCK, SAID ACIDULATE RESULTING FROM THEACIDULATION OF SAID FINE PHOSPHATE ROCK BY SAID SULFURIC ACID AND SAIDACIDULATE CHARACTERIZED AS A SUBSTANTIALLY DRY AND NON-STICKABLE DISCRETE PARTICULATE MASS; SUBSEQUENTLY ADDING WATER TO THE RESULTINGPOWDERY ACIDULATE IN QUANTITY SUFFICIENT TO DILUTE SAID ACID TO ABOUT 80PERCENT H2SO4, THE TOTAL QUANTITY OF WATER INTRODUCED INTO SAIDRESULTING POW-